Field of the Invention
This invention relates to generating oscillating signals and more particularly to supplying an oscillator signal along with information related to its accuracy relative to a predetermined frequency.
Description of the Related Art
Modern electronic systems such as radios, tuners, micro controller units (MCUs), typically include a phase locked loop (PLL) (or other circuits) capable of doing frequency translation inside the system. A frequency reference signal is often supplied to the system. In the past, the frequency reference signal supplied to the system had a pre-determined fixed frequency obtained by precise manufacturing of crystal resonators or by electronically correcting error in the frequency with a frequency translation circuit such as a PLL. Errors associated with variations in temperature were addressed by tuning the resonator load or adjusting the frequency translation ratios resulting in a fixed frequency output.
Increasingly, Micro Electrical Mechanical System (MEMS) based oscillators are being used to generate the reference signal. MEMS generally refers to an apparatus incorporating a mechanical structure capable of movement. MEMS resonators have potential to displace traditional crystal (quartz) resonators as a source for frequency reference signals in various electronic systems. MEMS resonators have many advantages such as smaller size, a manufacturing flow compatible with high volume semiconductor industry manufacturing processes, and lower cost. One drawback to utilizing MEMS-based oscillators relates to frequency tuning Unlike quartz resonators whose frequency can be precisely controlled by cutting, MEMS resonator frequency has intrinsic initial frequency inaccuracy due to manufacturing tolerances. Also, unlike a crystal oscillator whose frequency can be pulled (adjusted) by adding or subtracting capacitance on the resonator node, it is very difficult to pull MEMS resonator frequency to offset the manufacturing tolerances. A fractional-N phase-locked loop (PLL) is often used to correct the inaccurate MEMS frequency to a pre-determined accurate fixed frequency using the frequency translation ratio of the PLL. Similarly, temperature stabilized MEMS oscillators require frequency correction for temperature effects.
FIG. 1 shows a prior art system in which a MEMS resonator 101 is combined with the MEMS oscillator sustaining circuit 102 to provide a MEMS oscillator signal 106 to frequency correction PLL 108. Frequency correction PLL 108 is typically implemented as a fractional-N PLL. The frequency error correction circuit 112 supplies error information to the PLL 108 to allow the PLL to correct for initial frequency offset of the MEMS resonator and temperature effects by ensuring the PLL 108 has the correct frequency translation ratio. The frequency correction PLL 108 supplies an accurate frequency reference signal 116 that has a predetermined frequency known to the receiving system 130. The frequency reference signal 116 may be accurate to, e.g., 200 parts per billion or some other accuracy suitable for the receiving system. The receiving system utilizes the frequency reference signal 116 in frequency translation PLL 132 to generate a system clock 134 used by functional circuits 136 of the receiving system. The frequency translation PLL 132 translates the frequency reference signal 116 based on a desired frequency signal 138. That desired frequency signal may indicate to the frequency translation PLL 132 to multiply the reference frequency by, e.g., 117.6. Thus, the frequency translation PLL 132 may also be implemented as a fractional-N PLL to allow for non-integer ratios between the system clock 134 and the frequency reference signal 116.
However, there are multiple drawbacks to using a PLL in conjunction with a MEMS oscillator to generate the frequency reference signal 116. In particular, the PLL adds complexity, additional noise, and power consumption.